The option was called V.O.E. ( vacume operated exhaust ) . It was a 1970 option. It was pulled from the option sheet by GM after (The Humbler ) super bowl comercial in 1970. Check out the humbler comercial on youtube. It shows the voe operation. The voe knob is pulled and a vacum opens up the muffler chambers. Voe mufflers are being reproduced by the aftermarket. voe knobs and mounting plates are also reproduced.